1. Field of the Invention
The present invention relates to thermistor temperature sensing systems used to sense or measure a desired temperature or range of temperatures.
Generally speaking, the invention disclosed herein includes a unique thermistor temperature sensing system for detecting a limited range of temperatures and a means for extending previously linearized characteristics of a thermistor with respect to temperature where the thermistor is utilized in such a circuit for sensing a limited range of temperatures and it is desirable to extend the range of temperatures which may be accurately sensed.
3. Description of the Prior Art
It is well known in the art that the characteristics of a thermistor vary with temperature in a non-linear fashion and more particularly that the resistance of a thermistor decreases exponentially as the temperature of the thermistor increases. Accordingly, in order to utilize a thermistor as an accurate temperature sensing and/or measuring element in a temperature sensing system a means for linearizing the exponential characteristics of the thermistor is necessary. A well known technique for linearizing the characteristics of a thermistor over a limited range of temperatures is shown in FIG. 1. Such technique includes electrically coupling a single fixed resistor in series with the thermistor. This combination when coupled to a DC power supply produces a characteristic curve of current versus temperature for the thermistor such as is shown in FIG. 2. It is clear that for a limited range of temperatures L (FIG. 2) the current I passing through a thermistor which has been coupled in series with a resistor (as shown in FIG. 1) is a linear function of the thermistor temperature T. In fact, the point of optimum linearization of the current versus temperature characteristics will occur at a temperature T.sub.i representing the inflection point of the curve shown in FIG. 2.
Typical temperature sensing systems which utilize a thermistor as the sensor recognize that the voltage at a node joining the thermistor with the linearization resistor will be a linear function of temperature in a limited range of temperatures since the current is linearly increasing with temperature within such range of temperatures. Accordingly, the typical temperature sensing system utilizes a conventional comparator to compare the voltage at the node joining the thermistor to the linearization resistor to a voltage established by the setting of a linear potentiometer. When the voltage at the node substantially equals the voltage established by the setting of the potentiometer a signal is produced by the comparator which may be used to indicate that the temperature of the thermistor has reached the desired set point temperature.
However, the conventional temperature sensing system which utilizes a thermistor as the sensing element in the manner described above has the distinct disadvantages of only being accurate over a limited range of temperatures because of the limited ability to linearize the characteristics of the thermistor by the technique shown in FIG. 1, of requiring the use of a more expensive potentiometer, and the reference threshold voltage of the comparator is a function of the setting of the potentiometer. As shown in FIG. 2, at extreme operating temperatures of the thermistor the linear characteristics of the current versus temperature curve severely deteriorate and therefore a temperature being sensed by a sensing system utilizing a thermistor which is within the extreme temperature zones can not be measured accurately. It can therefore be seen that the conventional thermistor sensing system is limited to the measurement of temperatures falling within a very narrow band of temperatures where the band of temperatures is determined by the value of the linearizing resistor coupled in series with the thermistor and the physical parameters of the thermistor utilized as the sensing element.